Bone conduction hearing aids are essential for the rehabilitation of patients suffering from hearing losses for which traditional hearing aids are insufficient. Direct bone conduction hearing aids have a vibrating transducer that transmits vibrations directly to a fixture anchored in the bone, i.e. the skin does not take part in the transmission of the vibrations from the vibrator to the fixture in the bone. The most common type of such devices consists of an external hearing aid with a vibrating transducer which, through a coupling, is connected to a skin-penetrating abutment that has an interconnection to a screw shaped fixture anchored in the skull bone. A direct bone conduction hearing aid system may however be designed in other ways.
The fixture is usually made of titanium and is often designed with a flange to prevent the fixture from being pushed through the skull bone in case of a sudden accidental impact.
To insert the fixture into the skull bone a hole is first drilled in the bone. After that the fixture can be screwed into the hole directly since the fixture is traditionally equipped with cutting edges to prepare the thread in the bone. The bone shivers are collected in shiver spaces at the cutting edges on the fixture. If the bone is very hard, a screw tap may be used to prepare the thread or part of the thread before the fixture is inserted. The fixture is then integrated with the skull-bone. This process is called osseointegration. After around 3 months, the osseointegration is usually sufficiently strong so that the fixture can be loaded and used. To achieve a firm and stable anchoring for the hearing aid a strong osseointegration is essential. How well the fixture is anchored to the skull bone is determined by several factors such as the surface characteristics of the fixture, the mechanical design of the fixture and the area of the contact surface between the bone and the fixture. For the osseointegration process to be successful it is also important that the fixture is stable in the bone during the first 3 months when the osseointegration is established. The initial stability of the fixture in the bone is therefore also important for a successful treatment.
The shiver spaces, where the fixture is not in direct bone contact directly after fixture insertion, are also, after a successful osseointegration has taken place, filled with bone tissue that comes in direct contact with the fixture surface in these cavities.
To fixate the skin penetrating abutment to the fixture a connection screw is usually placed through the abutment and then screwed into a threaded hole in the fixture. It is important that this connection screw is sufficiently big to ensure a strong fixation of the abutment.
The thickness of the skull bone is usually between 3–5 mm and the thickness determines the appropriate length of the fixture. A wide fixture offers more contact with the bone but, on the other hand, if it is too wide and short it might be difficult to insert. Therefore, the diameter of the fixtures is usually in the range 3.5–5 mm. To ensure that the biomechanical and osseointegration properties of the fixture are functioning well, the pitch of the thread is often chosen to be in the range of 0.5 to 0.8 mm.
Existing fixtures used for anchoring direct bone conduction hearing aids have a depth of the thread of only around 0.3 mm. The depth of the thread on existing fixtures is less than 10% of the maximum outer diameter of the threaded portion of the fixture which limits the possible area of the contact surface between the fixture and the bone and limits the strength of the anchoring in the bone.
Due to the small depth of the thread on existing fixtures, the drilled hole in the bone has to have a diameter which is quite close to the inner diameter of the thread. Otherwise, there would be a low initial stability of the fixture in the bone. When inserting a fixture in a hole that has a diameter which is quite close to the inner diameter of the thread quite a lot of bone shivers are generated. These bone shivers must be collected mainly in the shivers space cavities at the cutting edges on the fixture. Therefore, these shiver space cavities must be quite big. However, since the fixture includes an inner hole for the connection of, for example, an abutment connection screw, it may be difficult to do the shiver spaces sufficiently deep without interfering with the inner hole in the fixture. This compromise leads to a less optimal design of existing fixtures.
Existing fixtures used for anchoring direct bone conduction hearing aids have a simple quite smooth machined titanium surface. The existing fixtures also have a very thin titanium oxide layer on its surface. These surface characteristics do not present the optimal properties to achieve maximum osseointegration.
Existing fixtures has also a flat surface on the side of the flange facing the threaded portion. A common problem that occurs when the fixture has osseointegrate is that the bone closest to the flange is resorbed which gives a lower stability of the fixture in the bone.
Due to the less optimal macroscopic and microscopic properties of existing fixtures, patients experience several clinical problems. One problem is fixtures that do not osseointegrate properly from the beginning so that the patient has to come back to the hospital to have a new surgical procedure performed. Another problem is that fixtures that have osseointegrated with the bone become loose in the bone due to mechanical load on the fixture. The mechanical load may for example be pulling or rotational forces. In this case a new surgery has also to be performed. The problem with existing fixtures is that the design and the thread profiles have mainly been copied from standard mechanical screws rather than being designed for its clinical, biomechanical and osseointeration purpose. Designing an anchoring system for a lifelong medical rehabilitation requires paying attention to the biological implant interactions with the bone tissue. This is a demanding development work where the lack of optimized key design parameters may lead to the need for patients coming in for surgical procedures, which could have been avoided if the fixture design would have been more favorable from a biomechanical and biological point of view. The existing fixture designs have and have always had problems with for example fixtures coming loose from the skull bone, due to an insufficient interaction with the bone tissue.
There is a need for a more effective anchoring and fixture arrangements that do not have the deficiencies described above.